1. Field of the Invention
This invention relates to methods and apparatus for improved detection, analysis, and discrimination of signals in receiving devices. In particular, these methods and apparatus reduce the undesirable effects of sharp signal pulses, called edges from strong signal emitters, allowing for greater dynamic ranges in detection.
2. Background of the Invention
Devices which receive and analyze a plurality of incoming signals through one or more detectors often process these signals to determine the number, direction, and strengths of various signal emitters within their vicinity. Radar and sonar receivers provide good examples of such detection devices. These receivers seek to locate and identify all emitters in their area, including both strong and weak signal sources. However, certain obstacles prevent precise and complete tracking. In particular, signals which turn suddenly on or off during reception and processing tend to drown out weaker signals, making their detection and discrimination much more difficult.
Many signal detection devices rely upon spectral analysis to identify and discriminate among a plurality of radar, sonar, or other electromagnetic signal sources. For data in digital form, special analysis through the use of a Fast Fourier Transform (FFT) yields the frequency spectrum which a particular signal source emits, effectively tagging individual sources by their frequencies. Fourier theory shows that any arbitrary signal or function, in this case a function of time, namely an electromagnetic wave, can be broken down or decomposed into a sum of individual sine waves of differing frequency.
The results of an FFT are often expressed in a power spectrum chart, graphing the amplitude weighting for each fourier sine wave component as a function of that component's frequency (sometimes called frequency bin or an FFT "filter"). One can reconstruct the original repeating signal function by multiplying each sine wave by its amplitude in the power spectrum and adding the resulting products. An FFT of a simple sine wave yields a single amplitude point on a power spectrum, at exactly that frequency. For an arbitrary signal in time, the power spectrum will generally be more complex, but will have a peak centered on the emitter's principal frequency.
Fourier transform theory assumes that the signal being analyzed has been "on" forever. If a signal that appears to be a clean repeating sine wave suddenly turns off or on during an FFT analysis, the power spectrum no longer provides a simple answer for the emitter frequency. This sudden turning on or off during transmission defines a signal "edge". The power spectrum of an edge no longer yields a peak for a single frequency. Instead, one requires an infinite number of sine wave frequencies to reconstruct the sudden discontinuity in the repeating wave. Hence, a power spectrum of an edge gives rather large amplitudes for a continuum of temporal frequencies. This broadened power spectrum constitutes a "special splatter" for the signal being analyzed. Such frequency or spectral splatter impairs detection of weaker radar signals, simply because the frequency components of the edge drown out a signal of weaker frequency. No method or apparatus has been available to reduce the spectral splatter of an edge, while still detecting the primary frequency of the strong emitter and also detecting the frequency of weaker signals.